In recent years, an increasing number of people suffer from depression. When patients suffering so-called masked depression without any psychotic manifestation are included, the total number of such people accounts for a significant proportion, and it is said that one out of every five Japanese will experience depression once in their lives.
The causes of depression still remain uncertain at this time, and are not limited to biological factors and personality factors. For example, the current severe changes in the social environment, the social structure where the weak are abandoned, and the trend for one-dimensional values eliminating heresy are involved in the increase of depression patients.
The therapy of depression includes: first of all, rest; next, drug therapy; and a combination thereof. However, most people are not really allowed to take a rest, and therefore they actually distract themselves with antidepressants.
There are various kinds of antidepressants from those called first generation including imipramine to those called fourth generation including the latest milnacipran. However, these antidepressants can cause side effects such as dry mouth, constipation, urination disorder and anuresis. In addition, there is concern about interaction with other drugs.
Meanwhile, examples of known agents containing a branched amino acid that is an active ingredient of the present invention include an agent for relieving muscular pain and muscle strain and stiffness by oral ingestion of a branched amino acid (see JP-A 2000-26289) and an agent for maintaining instantaneous or sustaining muscular power during exercise by oral ingestion of a branched amino acid (see JP-A 2000-26290). By focusing on the fact that branched amino acids such as leucine, isoleucine and valine have organ-specificity so that they can be utilized mainly in muscles and in tissues such as kidney other than liver, it is found that the branched amino acids are useful in ameliorating muscular pain and muscle stiffness or in maintaining instantaneous or sustaining muscular power during exercise. Also known is an agent for relieving fatigue in the central nervous system containing a branched amino acid (see WO2002/034257) or the like. The document elucidates the mechanism of fatigue in the central nervous system and simultaneously demonstrates that 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid that is a specific inhibitor of L-system transporter in the blood-brain barrier, can suppress the fatigues in the central nervous system, particularly almost completely when used in combination with a branched amino acid such as leucine, isoleucine or valine. In addition, a composition for improving brain cell metabolism (see JP-A 2-172915) and an anti-dementia drug (see JP-A 3-275631), both containing a branched amino acid, are known. However, these documents do not describe that branched amino acids have antidepressant activity.
Known pharmaceutical preparations containing branched amino acids include: a pharmaceutical jelly containing, as active ingredients, only branched amino acids having excellent flavor and excellent feel in swallowing, the jelly being administered in reduced dosage (see JP-A 2003-221330); a pharmaceutical dry syrup containing branched amino acids, a suspending agent and a surfactant, the dry syrup having improved flavor and feel in drinking, and keeping excellent suspensibility when suspended (see JP-A 2003-221329); and a chewable tablet containing, as active ingredients, branched amino acids, the tablet remaining uncolored during storage because of its excellent storage stability (see JP-A 2003-221327). However, none of these documents describes antidepressant activity.